1. Field of the Invention
The present invention relates to a charging apparatus for charging secondary batteries such as nickel cadmium batteries, and nickel hydrogen batteries.
2. Background Art
In the case of charging a battery pack such as a nickel cadmium battery or a nickel hydrogen battery used as a power source for a cordless tool etc., although the battery pack can be charged in a short time when charged with a large current, an amount of heat generated from the battery within the battery pack during charge becomes large and so the cycle life of the battery becomes shorter. Thus, a charging apparatus is proposed that charges a battery pack with a small charging current for a long time in order to suppress the heat amount of the battery.
On the other hand, another charging apparatus is proposed that charges a battery pack with a large charging current while cooling the battery pack by a cooling fan provided at a charging apparatus thereby to charge the battery pack in a short time with suppressing the heat amount of the battery pack during charge.
The aforesaid two charging apparatuses differ in their charging methods depending on whether or not the battery pack can cope with the forced cooling, that is, whether or not the battery pack is provided with a structure such as an air hole with respect to air blown from a cooling fan. In view of the fact that the charging method differs in this manner between the battery pack with a cooling device and the battery pack without a cooling device, a charging method is proposed which determines whether or not the rising rate of temperature of the battery at the time of start of charging is more than a predetermined value thereby to determine the non-existence or existence of the cooling effect on the battery pack due to the cooling fan, and an average charging current is made large when the cooling effect exists, whilst the average charging current is made small when the cooling effect does not exist (see U.S. Pat. No. 6,204,639, for example).
As a method of determining the full charge of a battery pack, there is proposed a dT/dt detection method which detects that a battery is fully charged when a battery temperature increasing rate calculated at some sampling interval increases by a predetermined value or more from the minimum value within a battery temperature increasing rate storing unit (see JP-A-6-113475, for example).
However, according to the charging method which selects the average charging current depending on the non-existence or existence of the cooling effect, there arises a problem that the cooling ability is not brought to the maximum and so the charging time can not be made shorter even for the battery pack a the cooling device.
For example, as shown in the charging characteristics of FIG. 1, in the case where a charging control method is employed which changes the charging current from a value I0 to a value I2 when the battery temperature reaches a predetermined temperature (45° C.) after the start of the charging, the charging time of the battery pack with a cooling device can be made shorter than that of the battery pack without a cooling device. However, since the charging time with the charging current I2 is long, the battery pack with a cooling device does not fully use the cooling ability thereof.
Further, it is unreasonable to determine the presence or non-presence of cooling effect by simply using the battery temperature increasing rate calculated at a sampling interval used for determining the full charge in the dT/dt detection method.
This will be explained with reference to FIGS. 2 and 3. FIGS. 2 and 3 are graphs showing changes of battery voltages, charging currents and a battery temperature changing rate (A/D conversion value) at a first sampling interval used for determining full charge at the time of charging the battery pack with a cooling device and the battery pack without a cooling device and showing changes of a battery temperature changing rate (A/D conversion value) at a second sampling interval (>the first sampling interval) used for determining the existence or non-existence of cooling effect, wherein a battery temperature K described in the abscissa represents a time where the battery temperature reaches a predetermined value K. A minimum battery temperature changing rate until the battery temperature reaches a predetermined battery temperature K is small in difference between the battery pack with a cooling device and the battery pack without a cooling device in the case of the battery temperature changing rate at the first sampling interval, but large indifference between the battery pack with a cooling device and the battery pack without a cooling device in the case of the battery temperature changing rate at the second sampling interval. In this manner, it is difficult to determine the existence or non-existence of cooling effect by simply using the battery temperature changing rate calculated at the sampling interval used for determining the full charge in the dT/dt detection method. Further, the detection of full charge may be delayed and so the battery may be overcharged at the time of detecting full charge based on the battery temperature changing rate at the second sampling interval.
Further, since the battery temperature changing rate from the start of charging differs depending on the battery temperature at the time of starting charge, it is unreasonable to determine the existence or non-existence of cooling effect to determine a charging current simply depending on whether or not the battery temperature changing rate at the time of starting charge is equal to or more than the predetermined value.